It seems what you’re referring to on your site is about a featured image within a widget on your front page. This actually isn’t related to the featured image on posts. In your case the correct code would be:

.mh-posts-focus-thumb img { width: auto; }

However, if you want to make use of featured images, you should upload images that meet at least the minimum requirements. You can learn more about that here:

Please also keep in mind that if you just want a list of posts on your front page, there is no need to create a static front page based on widgets (the widget you’re currently using is supposed to have a large thumbnail). Instead you could just set your front page to display latest posts in your WordPress dashboard under Settings => Reading.